This application is based upon and claims priority to Malaysian Patent Application No. PI 2000 6079, filed Dec. 21, 2000, which application is hereby expressly incorporated by reference.
This invention relates to improvements in cooling systems of a water-cooled engine and preferably, though not exclusively, to a cooling system to cool the upper regions of an interbore bridge of such an engine.
The narrow structure between two cylinders of a cylinder block is known as the interbore bridge. It has a high thermal concentration. This region has high surface to volume ratio, and easily be overheated if exposed to high heat sources. Heat sources can come from combustion within the cylinders, and also from the friction between the piston assembly and cylinder wall.
Problems are likely to occur once the surface temperature of the cylinder wall at the interbore bridge reaches 180xc2x0. At that temperature lubrication oil, especially mineral oil, will experience performance deterioration. Deterioration in the lubrication oil may cause friction between the piston and the cylinder wall to increase significantly. This will cause other problems such as, for example, piston scuffing, and excessive wear of the cylinder wall and piston rings.
Cylinder blocks made of aluminum will have additional problems once the temperature exceeds 220xc2x0 C. At that temperature and above, the aluminum weakens. Excessive bore distortion due to a thermally weakened structure, and thermal expansion, can be problematic to overall engine functionality. High thermal loading can also cause the structure to lose its original properties once the metal temperature is back to normal.
In order to overcome interbore overheating, many types of interbore cooling systems have been proposed including:
1) a full saw cut in the top surface;
2) double saw cuts in the top surface, one on each side of the water passage;
3) crossed-drilled passages from the top surface of stepped bore diameter extending from each end of the interbore bridge;
4) cast water passages; and
5) a water passage created by using a glass core.
Each system has its own advantages and disadvantages. There are many parameters to consider before choosing any particular design, and one particular design may work on one specific application, and not necessarily on others.
It is now a common practice that the one engine block can be used for many different applications. For some applications, the block is used for mass production engines of relatively low performance as well as limited production engines of high performance.
Generally, both maintain the maximum commonality of engine parts. The only parts that are completely different from one to another may be intake and exhaust systems, and camshafts.
To ensure the same cylinder block can be used for high and low performance engines with minor modification, a cylinder block with cylinder liners is one possible option. However, a high performance engine requires an interbore cooling system to cool the interbore bridge. On the other hand, a lower power output engine may not require interbore cooling.
One must therefore consider several options for interbore cooling. Cast water passages and cross-drilled passages are often rejected as, for example, an 8 mm interbore bridge has the liner-aluminum-liner arrangement of 2-4-2 mm. The thickness of the metal between the two liners is not enough to have a cast water passage, and insufficient for cross-drilled passages to pass through.
Interbore cooling options like a full saw cut, double saw cuts, and glass core are, however, available. A full saw cut is widely used for cylinder blocks with liners. The machining process is quite simple, except that the casting process must be accurate or 100% ultrasound is required in order to avoid improper load or damage to the saw during machining. Another problem with a full saw cut is sealing as it requires expensive gaskets. Leaking can also occur because the opening at the top deck requires proper sealing. Moreover, the opening weakens the interbore bridge and bore distortion is likely to occur during engine running due to thermal and mechanical loading.
With double saw cuts, both sides of the interbore bridge are saw cut in order to bring water flow closer to the hottest spot at the center of the interbore bridge. The cylinder head is also machined in order to link the water flow from the saw cuts at the cylinder block. This design requires long machining times, especially with the cylinder head.
The option of using a glass core has been proposed as it can create a water passage between the cylinder liners. The process is not yet in production. The process is also expensive, and must be strictly controlled. The glass core requires a high-speed water jet to remove it.
From a Computer Aided Engineering result, it has been determined that the center of the top region of the interbore bridge is the hottest region, and covers about 40% of the piston ring travel path. Heat flux starts to reduce dramatically after about 40% of the piston stroke. Therefore, cross-drilled passages are not at the hottest part of the interbore bridge and therefore are not totally effective in providing the cooling where it is required. Furthermore, they require complex and relatively expensive drilling operations.
It is therefore the principal object of the present invention to provide an interbore cooling system for water cooled engines to provide a relatively high level of cooling with a relatively low production costs and time.
With the above and other objects in mind, the present invention provides a cooling system for cooling an interbore bridge of a cylinder block of a water cooled engine, the interbore bridge having a top surface and a central region of minimum width; the cylinder block having a water jacket; the cooling system including at least one water passage extending from the top of the interbore bridge in, at or adjacent the central region to the water jacket.
Preferably, there are two water passages, one for each side of the central region; and the water jacket includes two flat surface through which each of the water passages pass, the flat surface being substantially normal to the water passages.
More preferably, the water passages are at an included angle to a vertical axis of the central region, the included angle being less than 90xc2x0, preferably from 3xc2x0 to 30xc2x0, more preferably from 5xc2x0 to 25xc2x0, or being 5xc2x0 or 25xc2x0.
Advantageously, each water passage is of constant diameter along its length, the constant diameter preferably being in the range of 1 mm to 3 mm, more preferably being 2 mm.
More advantageously, the water passages are not stepped and are formed by a drilling process which does not include a stepped drilling process.
The present invention also relates to a cylinder block, ladder frame, or bedplate including the above cooling system.
In another aspect, the present invention provides a method of forming at least one cooling passage in a cylinder block to cool an interbore bridge, the interbore bridge having a top surface and the cylinder block having a water jacket; the method including the steps of drilling at least one water passage to extend from the top surface in, at or adjacent a central region having minimum width, to the water jacket.
Preferably, there are two water passages, one for each side of the central region, and there are two flat surfaces formed on the water jacket through which each of the water passages pass, the flat surface being normal to the water passages.
Advantageously, the water passages are at an included angle to a vertical axis of the central region, the included angle being less than 90xc2x0, preferably 3xc2x0 to 30xc2x0, more preferably 5xc2x0 to 25xc2x0; or it may be 5xc2x0 or 25xc2x0.
Preferably, each water passage is of constant diameter along its length, more preferably in the range of 1 mm to 3 mm; or may be 2 mm.
More preferably, the water passages are not stepped and are drilled without using a stepped drilling process.